Many portable electronic devices such as cameras, cellular telephones, personal digital assistants (PDAs), MP3 players, computers and other devices include an imager for capturing images. One example of an imager is a complementary metal-oxide semiconductor (“CMOS”) imager. A CMOS imager includes a focal plane array of pixels, each one of the pixels including at least one photosensor overlying a substrate for accumulating photo-generated charge in the underlying portion of the substrate. Each pixel may include at least one electronic device such as a transistor for transferring charge from the photosensor to a storage region.
Each pixel has corresponding readout circuitry that includes at least a charge storage region connected to the gate of the output transistor, an output source follower transistor, a reset transistor for resetting the charge storage region to a predetermined charge level, and a row control transistor for selectively connecting the readout circuitry to a column line. The charge storage region may be constructed as a floating diffusion region. Each pixel may have independent readout circuitry, or may employ common element pixel architecture (CEPA), that may include multiple pixels sharing a single set of readout circuitry.
A pixel (including a photosensor), and its corresponding readout circuitry, are herein collectively referred to as a “pixel circuit.” In a CMOS imager, the active elements of a pixel circuit perform the necessary functions of: (1) photon to charge conversion; (2) accumulation of image charge; (3) resetting the storage region to a known state; (4) transfer of charge to the storage region accompanied by charge amplification; (5) selection of a pixel circuit for readout; and (6) output and amplification of a signal representing a reset level and pixel charge. Photo charge may be amplified when the charge moves from the initial charge accumulation region to the storage region. The charge at the storage region is typically converted to a pixel output voltage by a source follower output transistor.
FIG. 1 illustrates a typical four-transistor (4T) pixel circuit 100 utilized in a pixel array of an imager, such as a CMOS imager. The pixel circuit 100 includes a pixel having a photosensor 102 (e.g., a photosensor, photodiode, photogate, or photoconductor) and a transfer transistor 120. Pixel circuit 100 also includes readout circuitry, including a storage region configured as a floating diffusion region 106, a reset transistor 130, a source follower transistor 140, and a row select transistor 150. The at least one photosensor 102 is connected to the floating diffusion region 106 by the transfer transistor 120 when the transfer transistor 120 is activated by a transfer control signal TX. The reset transistor 130 is connected between the floating diffusion region 106 and an array pixel supply voltage VAA. A reset control signal RST is used to activate the reset transistor 130, which resets the floating diffusion region 106 to a predetermined reset voltage corresponding to the array pixel supply voltage VAA, as is known in the art.
The source follower transistor 140 has its gate connected to the floating diffusion region 106 and is connected between the array pixel supply voltage VAA and the row select transistor 150. The source follower transistor 140 converts the charge stored at the floating diffusion region 106 into an electrical output signal. The row select transistor 150 is controllable by a row select signal RS for selectively outputting an output signal VOPIX from the source follower transistor 140 onto column line 108. In a CMOS imager, two output signals are conventionally generated for each pixel circuit; one being a reset signal VOPIX—RST generated after the floating diffusion region 106 is reset, the other being an image or photo signal VOPIX—SIG generated after charges are transferred from the photosensor 102 to the floating diffusion region 106. This process is commonly referred to as “correlated double sampling” or “CDS”. Output signals VOPIX—RST, VOPIX—SIG are selectively stored in a sample and hold circuit (not shown).
Image sensors, such as an image sensor employing the conventional pixel circuit 100, have a characteristic dynamic range. Dynamic range refers to the range of incident light that can be accommodated by an image sensor in a single frame of pixel data. It is desirable to have an image sensor with a high dynamic range to image scenes that generate high dynamic range incident signals, such as indoor rooms with windows to the outside, outdoor scenes with mixed shadows and bright sunshine, night-time scenes combining artificial lighting and shadows, and many others.
The dynamic range for an image sensor is commonly defined as the ratio of its largest non-saturating signal to the standard deviation of its noise under dark conditions. The dynamic range is limited on an upper end by the charge saturation level of the sensor, and on a lower end by noise imposed limitations and/or quantization limits of the analog-to-digital converter used to produce the digital image. When the dynamic range of an image sensor is too small to accommodate the variations in light intensities of the imaged scene, e.g. by having a low saturation level, image distortion may occur.
A problem associated with charge generation in conventional pixel circuits occurs when the incident light captured and converted into charge during an integration period is greater than the charge storage capacity of the photosensor. In the event that a photosensor of a pixel circuit is exposed to an amount of incident light that generates a charge that exceeds the capacity of the photosensor, any additional photon-to-charge conversion will require some charge leakage to escape the photodiode region 102. Often times this leakage causes charges to migrate to adjacent pixel circuits causing cross-talk.
Additionally, when the charges generated during an integration period are output from the photosensor during transfer to the floating diffusion region 106, a small amount of residual charge can remain in the photosensor. The residual charge causes charge accumulation excess in a subsequent captured image and may cause the photosensor to more quickly exceed its maximum capacity, thereby causing excess charge to overflow to adjacent pixels. This undesirable phenomenon of charge overflow at the photosensor is known as blooming and can result in a number of vertical and/or horizontal streaks in the resultant output image.
One solution that has been proposed to this blooming problem is a multiple exposure mode, where charge generated in a photosensor region during multiple exposure periods are sampled and combined. For example, a pixel can be operated in a double exposure mode, where a first charge V1 generated in a photosensor region (such as, for example, photodiode 102 of FIG. 1) during a first exposure period T1 is output via the pixel circuit's output circuitry during a first sampling operation. The photosensor region is then reset, and a second charge V2 generated in the photosensor region during a second exposure period T2 is output via the pixel circuit's output circuitry during a second sampling operation. While a multiple exposure operation allows for a higher dynamic range of pixel circuit 100, the multiple cycles of charge generation and sampling cause a reduced frame rate. Furthermore, line buffers to store the multiple sampled charges are required, resulting in a higher required die size and/or a lower fill-rate of the pixel circuit.
Yet another solution that has been proposed is a lateral overflow mode, where photo-generated charge is generated in a photoconversion region (such as, for example, photodiode 102), and if the photo-generated charge exceeds a predetermined threshold of the photoconversion region, an excess portion of the photo-generated charge is transferred to a floating diffusion region of the pixel circuit (for example, floating diffusion region 106 of FIG. 1) after a first exposure period T1. The excess portion may be transferred by applying a “soft transfer” signal to a gate of a transfer transistor separating the photoconversion region from the floating diffusion region (e.g., transfer transistor 120 of FIG. 1), where the soft transfer signal is a voltage that is less than the voltage required to completely transfer charge across the transfer transistor.
The excess charge may be discarded by resetting the floating diffusion region (such as by applying a reset signal to reset transistor 130 in FIG. 1), or, alternatively, may be sampled. The remaining charge generated during first exposure period T1, as well as any additional charge generated during a second exposure period T2, is transferred into the floating diffusion region and sampled to generate an output pixel signal VOPIX—SIG. Regardless of whether the excess charge V1 is discarded or sampled, to account for the excess charge, the output pixel signal VOPIX—SIG must be calculated according to a “knee point” of the pixel circuit when operated in lateral overflow mode, the knee point being a factor of the soft-transfer signal and/or the resultant threshold voltage VTH of the transfer transistor. While a lateral transfer mode increases the dynamic range of the pixel, the knee point (correlating to the threshold voltage VTH) of the pixel circuit must be calibrated, requiring an additional readout cycle of the pixel circuit, and thus resulting in a reduced frame rate and/or added noise.
Yet another solution that has been suggested to overcome the above problems, as shown in FIG. 2 and as described, for example, in U.S. Pat. No. 7,238,977 B2, assigned to Micron Technology, Inc, the disclosure of which is incorporated herein by reference in its entirety, is to provide a pixel circuit 200 with an anti-blooming transistor 260. As shown in FIG. 2, pixel circuit 200 is similar to the 4T pixel circuit 100 of FIG. 1, but has an additional transistor 260. During an integration period for the pixel circuit 200, when the photoconversion region 102 (which may be any of a photosensor, photodiode, photogate, or photoconductor) becomes saturated with charge, the anti-blooming (AB) transistor 260 transfers some of the excess charge to a drain area 262 associated with the AB transistor 260. Drain area 262 may be, for example, connected to a pixel voltage VAA, and the excess charge is discarded into the drain area 262 of the AB transistor 260 without being sampled. The proposed design of pixel circuit 200 is effective for increasing the dynamic range over the conventional pixel circuit 100 (FIG. 1), however, the proposed pixel circuit 200 also has drawbacks. Because the excess charge is not sampled, if it is to be accounted for at all, the read out voltage VOPIX—SIG must account for the excess charge transferred across drain area 262 of the AB transistor 260. CMOS transistors, such as AB transistor 260, have a high deviation in threshold voltage from wafer to wafer, and often from transistor to transistor. This deviation leads to an uncertainty in the amount of charge stored from pixel circuit to pixel circuit since the threshold voltage of each transistor, including the anti-blooming transistor, could vary. The variance of charge storage from pixel circuit to pixel circuit leads to fixed pattern noise (FPN) in an imager array resulting in diminished image quality because of the non-uniformity of barrier heights of the transistor 260 from pixel to pixel.
An optimal pixel circuit has a high dynamic range with a predictable response, a low die size and high fill-rate, potential for a high frame-rate, and low noise. There is needed, therefore, a pixel circuit having improved saturation response and lower potential for blooming, but with reduced potential for the other effects caused by the previously proposed solutions described above.